mysql中innodb表中count()优化
发布:smiling 来源: PHP粉丝网 添加日期:2015-04-17 16:29:07 浏览: 评论:0
count()是用来统计数据表中所有记录的一个函数了,但在此函数在innodb中性能不怎么样了,下面我们来看看mysql中innodb表中count()优化,希望例子对各位有帮助.
起因:在innodb表上做count(*)统计实在是太慢了,因此想办法看能不能再快点.
现象:先来看几个测试案例,如下.
一、sbtest 表上的测试.
- show create table sbtest\G
- *************************** 1. row ***************************
- Table: sbtest
- Create Table: CREATE TABLE `sbtest` (
- `aid` bigint(20) unsigned NOT NULL auto_increment,
- `id` int(10) unsigned NOT NULL default '0',
- `k` int(10) unsigned NOT NULL default '0',
- `c` char(120) NOT NULL default '',
- `pad` char(60) NOT NULL default '',
- PRIMARY KEY (`aid`),
- KEY `k` (`k`),
- KEY `id` (`id`)
- ) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=latin1
- show index from sbtest;
- +--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
- | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | --phpfensi.com
- +--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
- | sbtest | 0 | PRIMARY | 1 | aid | A | 1000099 | NULL | NULL | | BTREE | |
- | sbtest | 1 | k | 1 | k | A | 18 | NULL | NULL | | BTREE | |
- | sbtest | 1 | id | 1 | id | A | 1000099 | NULL | NULL | | BTREE | |
- +--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
填充了 100万条 记录.
1、直接 count(*)
- explain SELECT COUNT(*) FROM sbtest;
- +----+-------------+--------+-------+---------------+---------+---------+------+---------+-------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+--------+-------+---------------+---------+---------+------+---------+-------------+
- | 1 | SIMPLE | sbtest | index | NULL | PRIMARY | 8 | NULL | 1000099 | Using index |
- +----+-------------+--------+-------+---------------+---------+---------+------+---------+-------------+
- SELECT COUNT(*) FROM sbtest;
- +----------+
- | COUNT(*) |
- +----------+
- | 1000000 |
- +----------+
- 1 row in set (1.42 sec)
可以看到,如果不加任何条件,那么优化器优先采用 primary key 来进行扫描.
2、count(*) 使用 primary key 字段做条件.
- explain SELECT COUNT(*) FROM sbtest WHERE aid>=0;
- +----+-------------+--------+-------+---------------+---------+---------+------+--------+--------------------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+--------+-------+---------------+---------+---------+------+--------+--------------------------+
- | 1 | SIMPLE | sbtest | range | PRIMARY | PRIMARY | 8 | NULL | 485600 | Using where; Using index |
- +----+-------------+--------+-------+---------------+---------+---------+------+--------+--------------------------+
- SELECT COUNT(*) FROM sbtest WHERE aid>=0;
- +----------+
- | COUNT(*) |
- +----------+
- | 1000000 |
- +----------+
- 1 row in set (1.39 sec)
可以看到,尽管优化器认为只需要扫描 485600 条记录(其实是索引),比刚才少多了,但其实仍然要做全表(索引)扫描,因此耗时和第一种相当.
3、count(*) 使用 secondary index 字段做条件
- explain SELECT COUNT(*) FROM sbtest WHERE id>=0;
- +----+-------------+--------+-------+---------------+------+---------+------+--------+--------------------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+--------+-------+---------------+------+---------+------+--------+--------------------------+
- | 1 | SIMPLE | sbtest | range | id | id | 4 | NULL | 500049 | Using where; Using index |
- +----+-------------+--------+-------+---------------+------+---------+------+--------+--------------------------+
- SELECT COUNT(*) FROM sbtest WHERE id>=0;
- +----------+
- | COUNT(*) |
- +----------+
- | 1000000 |
- +----------+
- 1 row in set (0.43 sec)
可以看到,采用这种方式查询会非常快,有人也许会问了,会不会是因为 id 字段的长度比 aid 字段的长度来的小,导致它扫描起来比较快呢?先不着急下结论,咱们来看看下面的测试例子.
二、sbtest1 表上的测试
- show create table sbtest1\G
- *************************** 1. row ***************************
- Table: sbtest1
- Create Table: CREATE TABLE `sbtest1` (
- `aid` int(10) unsigned NOT NULL AUTO_INCREMENT,
- `id` bigint(20) unsigned NOT NULL DEFAULT '0',
- `k` int(10) unsigned NOT NULL DEFAULT '0',
- `c` char(120) NOT NULL DEFAULT '',
- `pad` char(60) NOT NULL DEFAULT '',
- PRIMARY KEY (`aid`),
- KEY `k` (`k`),
- KEY `id` (`id`)
- ) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=latin1
- show index from sbtest1;
- +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
- | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
- +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
- | sbtest1 | 0 | PRIMARY | 1 | aid | A | 1000099 | NULL | NULL | | BTREE | |
- | sbtest1 | 1 | k | 1 | k | A | 18 | NULL | NULL | | BTREE | |
- | sbtest1 | 1 | id | 1 | id | A | 1000099 | NULL | NULL | | BTREE | |
- +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
这个表里,把 aid 和 id 的字段长度调换了一下,也填充了 1000万条记录.
1、直接 count(*).
- explain SELECT COUNT(*) FROM sbtest1;
- +----+-------------+---------+-------+---------------+---------+---------+------+---------+-------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+---------+-------+---------------+---------+---------+------+---------+-------------+
- | 1 | SIMPLE | sbtest1 | index | NULL | PRIMARY | 4 | NULL | 1000099 | Using index |
- +----+-------------+---------+-------+---------------+---------+---------+------+---------+-------------+
- SELECT COUNT(*) FROM sbtest1;
- +----------+
- | COUNT(*) |
- +----------+
- | 1000000 |
- +----------+
- 1 row in set (1.42 sec)
可以看到,如果不加任何条件,那么优化器优先采用 primary key 来进行扫描.
2、count(*) 使用 primary key 字段做条件.
- explain SELECT COUNT(*) FROM sbtest1 WHERE aid>=0;
- +----+-------------+---------+-------+---------------+---------+---------+------+--------+--------------------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+---------+-------+---------------+---------+---------+------+--------+--------------------------+
- | 1 | SIMPLE | sbtest1 | range | PRIMARY | PRIMARY | 4 | NULL | 316200 | Using where; Using index |
- +----+-------------+---------+-------+---------------+---------+---------+------+--------+--------------------------+
- 1 row in set (0.00 sec)
- SELECT COUNT(*) FROM sbtest1 WHERE aid>=0;
- +----------+
- | COUNT(*) |
- +----------+
- | 1000000 |
- +----------+
- 1 row in set (1.42 sec)
可以看到,尽管优化器认为只需要扫描 485600 条记录(其实是索引),比刚才少多了,但其实仍然要做全表(索引)扫描,因此耗时和第一种相当.
3、count(*) 使用 secondary index 字段做条件.
- explain SELECT COUNT(*) FROM sbtest1 WHERE id>=0;
- +----+-------------+---------+-------+---------------+------+---------+------+--------+--------------------------+
- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
- +----+-------------+---------+-------+---------------+------+---------+------+--------+--------------------------+
- | 1 | SIMPLE | sbtest1 | range | id | id | 8 | NULL | 500049 | Using where; Using index |
- +----+-------------+---------+-------+---------------+------+---------+------+--------+--------------------------+
- 1 row in set (0.00 sec)
- SELECT COUNT(*) FROM sbtest1 WHERE id>=0;
- +----------+
- | COUNT(*) |
- +----------+
- | 1000000 |
- +----------+
- 1 row in set (0.45 sec)
可以看到,采用这种方式查询会非常快,上面的所有测试,均在 mysql 5.1.24 环境下通过,并且每次查询前都重启了 mysqld.
可以看到,把 aid 和 id 的长度调换之后,采用 secondary index 查询仍然是要比用 primary key 查询来的快很多。看来主要不是字段长度引起的索引扫描快慢,而是采用 primary key 以及 secondary index 引起的区别,那么,为什么用 secondary index 扫描反而比 primary key 扫描来的要快呢?我们就需要了解innodb的 clustered index 和secondary index 之间的区别了.
innodb 的 clustered index 是把 primary key 以及 row data 保存在一起的,而 secondary index 则是单独存放,然后有个指针指向 primary key,因此,需要进行 count(*) 统计表记录总数时,利用 secondary index 扫描起来,显然更快,而primary key则主要在扫描索引,同时要返回结果记录时的作用较大,例如:
SELECT * FROM sbtest WHERE aid = xxx;
那既然是使用 secondary index 会比 primary key 更快,为何优化器却优先选择 primary key 来扫描呢,Heikki Tuuri 的回答是:
- in the example table, the secondary index is inserted into in a perfect order! That is
- very unusual. Normally the secondary index would be fragmented, causing random disk I/O,
- and the scan would be slower than in the primary index.
- I am changing this to a feature request: keep 'clustering ratio' statistics on a secondary
- index and do the scan there if the order is almost the same as in the primary index. I
- doubt this feature will ever be implemented, though.
Tags: innodb表 count()优化
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